Recently, the valve mechanisms of internal combustion engines have been required to bear heavy running loads. In particular, sliding parts, such as a camshafts and rocker arms, have been required to stand up against high plane pressures. Chromium-containing iron-base sintered alloys have been utilized in sliding parts not only to meet the aforementioned requirements but also to reduce the weight of the parts.
Such alloys are disclosed by JP A No. 54-62108, JP A No. 56-123353 (corresponding to U.S. Pat. No. 4,388,114) and JP A No. 58-37158. The alloy disclosed in 54-62108, contain, by weight, Cr, 8.0-30.0%; C, 0.5-4.0%; P, 0.2-0.3%, the balance being Fe. A problem arises with this alloy when the chromium exceeds 20.0% since the chromium-carbide grows coarser and harder which damages to opposing sliding parts. Another problem that arises is that it is too hard to be machined. The alloy of disclosed in No. 56-12353, contains, by weight, Cr, 2.5-7.5%; Cu, 1.0-5.0%; C, 1.5-3.5%; P, 0.2-0.8%; Si, 0.5-2.0%; Mn, 0.1-3.0%; Mo, less than 3.0%, the balance being Fe. This alloy is less shrinkable even when sintered at a liquid-phase since it contains more than 1% of copper. Thus, it is unavailable for fabricating the fitting members of a camshaft, such as cam lobes and the like, which are constrictively jointed to the shaft after being loosely mounted on the same. The alloy disclosed in No. 58-37158, contains, by weight, Cr, 2.5-25.0%; C, 1.5-3.5%; Mn, 0.1-3.0%; P, 0.1-0.8%; Cu, 1.0-5.0%; Si, 0.5-2.0%; Mo, less than 3.0%; S, 0.5-3.0%; Pb, 1.0-5.0%; the balance being Fe. This alloy has an advantage since copper is effective in preventing the growth of coarse chromium-carbide. However, it is relatively brittle because it contains sulphide and lead.
The present invention is intended to provide a chromium-containing iron-base sintered alloy that is superior in machinability and suitable for fabricating cam lobes and the like which are constrictively bonded to a shaft by liquid-phase sintering after being loosely mounted on the same shaft.